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Field tests show that the Next Generation Concrete Surface is competitive with the quietest pavements available.

In the past decade or so, environmental noise has become a major issue in many parts of the world, including the United States. Stakeholders ranging from the general public to the roadbuilding industry have expressed concerns over highway noise. Recognizing that highways are a factor in urban noise, the concrete pavement industry set out to research and develop quieter pavements without compromising performance, durability, safety or other inherent benefits of concrete.

NGCS pavements have been placed with either a single-pass or double-pass operation by the grinder, and both work equally well.

In recent years, the concrete pavement industry has developed a Next Generation Concrete Surface (NGCS). With field tests indicating sound levels of 99 to 101 decibels, these diamond-ground surfaces are producing results that are very competitive with the quietest pavements available, says Larry Scofield, P.E., Director of Pavement Innovation for the American Concrete Pavement Association (ACPA).

Sound testing is conducted using the On Board Sound Intensity (OBSI) method, which allows assessments of acoustic performance of pavements over time. The method was developed by General Motors and introduced to the highway community by the California Department of Transportation. More about the NGCS follows:

The background

Research showed that much of the perceived problem with concrete pavement’s noise results from a pure tone, or “whine” that occurs when noise with a certain discrete frequency emanates from the tire/pavement interface. Almost always, those frequencies were set with concrete pavement by uniformly-spaced transverse tining.

In an effort to improve pavement safety, the FHWA in the late 1970s had mandated transverse tining, and many states complied. Then in 2005, FHWA dropped the requirement for transverse tining, and opened the door to other concrete pavement texture treatments. California, for example, had used longitudinal tining for more than 30 years and reported few if any problems with it.

Recognizing the worldwide interest in quiet pavements, in 2004 ACPA, with support from the cement industry, developed a program to research the noise characteristics of concrete pavements. A primary objective was the evaluation and development of a quieter concrete pavement surface. Also providing support for the program were the International Grooving and Grinding Association (IGGA) and several of its members.

Purdue University’s Ray W. Herrick Laboratories conducted the research using its Tire Pavement Test Apparatus (TPTA). The machine consists of a 38,000-pound, 12-foot-diameter drum that makes it possible to test numerous pavement textures and compositions in combination with various tire designs. Six curved test sections of pavement fit together to form a circle around the vertical edge of the drum. Two tires, mounted on opposite ends of a beam, are then rolled over the test samples while microphones and other sensors record data. The TPTA has been described as a “noise microscope” for tire-pavement noise. Tire speeds of up to 30 mph can be tested.

Scofield says the Purdue diamond grinding research was based on theories that the blade- and/or spacer-widths might be the keys to a more quiet pavement surface. But after evaluating the range of blade and spacer widths requested by the industry, Purdue reported that no unique relationship could be found between sound levels and spacer width, blade width and spacer/blade configuration. Instead, it appeared that the controlling variable – where sound was concerned – was the variability in the fin profile height resulting from the grinding process. The fins are the tiny vertical ridges that appear on a diamond-ground concrete surface.

Textures with low variability were quieter than textures with high variability. In conventional diamond grinding, the resulting fin variability is influenced by the blade/spacer configuration, the concrete mixture, aggregate type, pavement condition, equipment set-up and more. Because the fin variability occurs in the field, it is difficult to adjust in a laboratory setting. Researchers decided to grind a pavement smooth, then impart additional texture by grooving, which provided an exact fin profile that could be controlled and predicted.

An epiphany

A conventional diamond-ground surface produces texture in the upward or positive direction, while the texture tested at Purdue produces texture in the downward or negative direction. “The texture, which later was called the Next Generation Concrete Surface (NGCS), was desirable from the standpoint that it was more of a ‘manufactured surface’ and thus could be controlled as necessary on an experimental basis,” says Scofield.

The Next Generation Concrete Surface is competitive with the quietest pavements available.

“When these new textures were tested on the TPTA, they produced the quietest diamond ground surfaces,” Scofield says. “This was an epiphany in the research because it verified, for the first time, what the controlling factor was for tire-pavement noise generation of diamond-ground surfaces.

“The point of the NGCS surface is to limit positive or upward texture,” Scofield explains. “The NGCS texture is designed to develop good macro-texture through ‘negative’ or downward texture (grooves). We want to have both good microtexture (the texture on top of the lands), and good macrotexture, which is developed primarily through the grooves.

“The NGCS is quieter because it relies on negative texture and not positive texture,” Scofield continues. “Since it is more of a manufactured surface, it can control the fin profile to a greater degree than previously possible. Purdue University determined that the fin profile is the critical element in noise generation.”

A new reality

The epiphany in research was soon confronted by reality, Scofield says. Research showed that the flush-ground-then-grooved texture could produce a quieter pavement. But the research could not verify whether such a texture could be constructed with conventional equipment in the field.

Next, in response to industry representatives, researchers developed two methods of reproducing the NGCS. The first was a grinding head configuration that used three smaller blades stacked between two taller blades. That pattern was repeated across the grinding head. That way, in one pass the head could grind the surface smooth and also groove it on approximately ½-inch centers in one pass of the machine. The smaller blades would flush grind the specimen and provide microtexture while the taller blades created grooves.

The second grinding configuration used the same smaller blades to flush grind the pavement in one pass. Next a second pass using taller blades with spacers created the grooves, similar to what was constructed with the single-pass operation.

That way, contractors could choose either option – the single pass or the double pass – in field construction. Some industry representatives thought the single-pass operation would cause excessive blade wear and have the potential for ruining the head and blades. Many believed the two-stage process would be required. Today, this is not a concern. NGCS pavements have been placed with a single-pass or double-pass operation, and both work equally well.

Field trials

The opportunity to construct field test sections became a reality when the Minnesota DOT allowed construction of test sections at the MnROAD Low Volume Road Test Cell Number 37 as part of an FHWA pooled fund effort. The two Purdue surfaces were to be compared to a conventional diamond grinding surface to assist in determining the benefit achieved by controlling the fin profile. So, there was a need to build three test surfaces.

This first-pass, flush-ground surface on I-355 in the Chicago area shows up as the whiter strip on the pavement.

Findings validated that the newly-developed surface was quieter, at the time of construction, than the conventional diamond ground texture. And, findings showed that the Purdue TPTA results could be reproduced in the field using conventional equipment. But because those were not full-width text sections, the next step was to construct a full-width test section using a conventional diamond grinding machine.

The first opportunity to construct a full lane-width test section occurred on Interstate 355 in the Chicago area. In October 2007, both a conventional diamond-ground test section and an NGCS were built on the I-355 tollway. The sections were 1,200 feet long and one lane wide.

Scott Eilken is the owner of Quality Saw and Seal, the diamond grinding contractor for the NGCS section at I-355. He is an ACPA member, a board member of IGGA, and was instrumental in writing the specifications for NGCS.

“On I-355 we did one pass to flush grind the surface, and the second pass as surface grooving,” Eilken recalls. “When we first tested it, the surface produced just 99 dB(A). We were one of the first concrete pavements in the nation to get below 100 dB(A), so it worked pretty well.”

The next opportunity to build test sections occurred at MnROAD’s Interstate 94. A two-lane wide by 500-foot-long section of NGCS was constructed in a single-pass operation on a 14-year-old random transverse-tined pavement in October 2007.

With the successful placement and performance of the two mainline sections, the ACPA officially named the texture as the Next Generation Concrete Surface (NGCS). The name describes a category of textures that evolve for both new construction and rehabilitation of existing surfaces.

“The desirable characteristics of such textures will be predominately negative texture coupled with good microtexture and excellent macrotexture,” says Scofield. NCGS can now be found at seven sites in five states. There are three test sections near MnROAD; one in suburban Chicago; one in Norman Okla.; one near Abilene, Kansas; and one near Omro, Wis.

Other test sections have been established this year in Washington state and in Arizona.

All surfaces are still performing as intended. As of 2009, ACPA’s figures show that OBSI testing was conducted on 288 pavement sections consisting of 126,720 lineal feet (24 miles) of concrete pavements across North America. The surfaces were evaluated with two goals in mind; first, to benchmark current surface texturing practices, and second, to develop insight into the acoustic longevity of textures. The acoustic longevity will become increasingly important as quiet pavement technology becomes integrated into noise mitigation.

Recent sound results from the MnROAD test sections indicate that NGCS pavements are running in the range of 99 dB(A) to 101 dB(A). By comparison, conventional diamond ground sections ranged up to 104 dB(A).

One test section in Kansas shows that an NGCS surfaces has produced 99 dB(A) and 100 dB(A) in two different tests. By comparison, an Astro-Turf drag surface ranges up to 102.5 dB(A) and an exposed aggregate surface ranges up to 104.5 dB(A).

A renewable surface

At MnROAD, Diamond Surface Inc. constructed a surface casually called NGCS LITE, which was designed as a renewable surface. It was developed to provide additional microtexture on existing NGCS surfaces if the need arose to do so. With the large land size (between the grooves) of the NGCS surface, the texture wear has been assumed to be less than for a conventional diamond-ground surface. So, NGCS is expected to have a long life by comparison. v

The M-E PDG can account for numerous variables in concrete pavement design

In its simplest terms, concrete pavement design optimization considers the performance benefits of various components versus their cost.

Design optimization can be thought of in various ways, says Michael Ayers, PhD, Director of Education for Design and Construction at the American Concrete Pavement Association (ACPA). Those include:

Achieving long life;

Lowering initial cost;

Minimizing maintenance and rehabilitation costs; and

Developing a sustainable, environmentally-sound pavement system.

Until recently, pavement designers were mostly limited to the 1993 AASHTO Pavement Design Procedure. Although many agencies are still using the 1993 protocol, it has limited inputs and is not ideal for optimizing pavement design.

By contrast, the current AASHTO Interim Mechanistic-Empirical Pavement Design Guide (M-E PDG) has many more input parameters, which allow designers greater influence over pavement performance. The M-E PDG combines empirical or observed pavement performance data from a number of sources – primarily the Long-Term Pavement Performance Studies done under the Strategic Highway Research Program, and mechanistically-calculated pavement response parameters.

“When you combine those two elements, it gives you the flexibility to account for various types of design optimization,” says Ayers. The variables to be considered depend on the design method used in the analysis. The M-E PDG has the capacity to consider support conditions (including subbases for concrete pavement), concrete materials and properties, load transfer (both longitudinal and transverse), and numerous other criteria.

ACPA provides training programs in which designs are generated using the AASHTO 1993 guide, M-E PDG and StreetPave software, and are optimized for performance. Ayers says slab thickness is often used as a basis for comparison between design elements. However, says Ayers, it is preferable to compare estimated costs for the overall pavement structure including the subbase, load transfer, slab configuration, etc.

To establish a baseline design using AASHTO 1993, Ayers established the following inputs for an example:

Traffic: 20 million 18-kip rigid Equivalent Single Axle Loadings

Reliability: 90 percent

Concrete Modulus of Rupture: 600 psi

Concrete Modulus of Elasticity: 4.050 million psi

Load transfer coefficient: 3.2

Drainage coefficient: 1.0

Initial serviceability: 4.5

Terminal Serviceability: 2.5

Initial and final serviceability reflect the initial construction quality (primarily smoothness) and the end of the design life of the roadway (the point at which major rehabilitation or reconstruction is required).

With those inputs, AASHTO 1993 produces a calculated slab thickness of 11.5 inches. With AASHTO 1993, concrete strength and load transfer are important parameters. However, specific guidance as to the configuration of load transfer is not provided. Concrete durability and dimensional stability issues are also not addressed.

Many important design elements cannot be accounted for using AASHTO 1993. Simply altering traffic levels, reliability, support conditions, and levels of serviceability is not a true design optimization strategy, Ayers says.

Inside the M-E PDG

In 2008, the M-E PDG was adopted as an Interim Design Procedure by AASHTO. Full implementation by the states will take a number of years, and some states may not adopt it. The “final” version of the program, referred to as DARWin ME, is currently under development. However, the research grade software is available as a free download at (http://trb.org/mepdg/home.htm).

Ideally, optimization should be conducted based on a state or regional calibration of the M-E PDG. State or regional calibrations take into account such factors as climate and locally available materials.

The bases for comparison among the various design features are the three failure criteria for concrete pavements in the M-E PDG. The three criteria include transverse slab cracking, joint faulting and smoothness as determined by the International Roughness Index (IRI).

In his example of how M-E PDG works, Ayers includes the following variables:

Concrete strength;

Coefficient of thermal expansion of the concrete;

Subbase type and thickness;

Dowel bar size;

Edge support; and

Joint spacing.

30-year design

Failure criteria was set to the default values for cracking, faulting and IRI. Slab cracking was set at 15 percent; a faulting threshold of 0.12 inches was used; and terminal IRI was set at 172 inches per mile.

Those values, Ayers emphasizes, can have a significant effect on your final design. Establishing realistic failure values for a specific project is a key to successful use of the M-E PDG software.

Two primary climatic zones were analyzed in the example: Wet/freeze in Chicago, Illinois; and dry/no freeze in Phoenix, Arizona. Traffic was based on 5,000 AADT, and M-E PDG default values were used for traffic variables. Fully 100 percent of the design traffic was allocated to the design lane. The compound annual growth rate was fixed at 2 percent, and the design period was 30 years.

Next, Ayers chose suitable values for: soil type; granular subbases; concrete properties (modulus of rupture and coefficient of thermal expansion); dowel bar diameters and spacing according to ACPA guidelines; and edge support. A number of pavement configurations were evaluated, including the following: a 12-foot lane with no shoulder; a 13-foot widened lane; a 14-foot widened lane, and a tied concrete shoulder.

Graphs were developed illustrating the climatic effects on estimated transverse cracking, estimated faulting, and estimated IRI. In each case, the independent variable is shown as the slab thickness, and the failure mode is plotted as the dependent variable.

Similarly, the effects of base type on estimated transverse cracking, faulting, and IRI were plotted for various base types. Again, the independent variable is the slab thickness and the failure mode is shown on the vertical axis. For example, at a slab thickness of 10 inches, using a granular base in Chicago with 50-percent reliability, cracked slabs go to zero.

The effects of various modulus of rupture values, coefficients of thermal expansion, dowel diameters and spacing, and edge support were plotted to show what happens to the various failure modes for varying slab thicknesses.

Ayers says the next steps are to establish unit costs for the most likely design elements such as dowels, widened lane, treated versus untreated subbase type, and so forth. Designs that meet specified criteria – specifications, design standards for the type of pavement structure being constructed, materials availability, etc. – are selected. An economic analysis is then conducted to select the “best” designs. The final step is to select the least-cost, best-performing option, and rerun the analysis on the selected design for verification.

The results

In Ayers’ trial runs of the ME-PDG, design thicknesses ranging from 8 inches to 12 inches met the established criteria depending on the design elements selected. An economic analysis would determine the optimal design elements to specify in the final design. For instance, the use of a 13-foot widened lane significantly improved performance and thereby lowered the required slab thickness. The economic analysis would then determine if the added cost of the widened lane was offset by the decrease in slab thickness and difference in anticipated performance. A similar analysis would be performed for each critical design element. The process can become somewhat complex when multiple variables are analyzed simultaneously, according to Ayers.

Conclusions

Design optimization can lead to cost savings and enhanced performance if correctly applied. In order to generate realistic design options, a relatively sophisticated design procedure such as the ME-PDG should be used. Although it is possible to generate comparable designs with the AASHTO 93 Guide, the limited number of variables and sensitivities make comparing options difficult.

Implementation of the ME-PDG is an important step in developing realistic comparisons between design options.v